This invention is intended to be used for the generation of position-coding patterns. A position-coding pattern is constructed of a large number of symbols which are printed on a base. By recording optically a particular minimum number of symbols with a reading device, the absolute position in the pattern of these symbols and thereby of the reading device can be detected.
Such a pattern can be used in various ways. For example, a reading device can be integrated into an ink pen that is used to write on the base. At the same time as the user writes text or draws a figure on the base, the movement of the pen over the base can be recorded as a sequence of positions. A digital copy of what the user wrote on the base is thereby created, without the use of a conventional scanner.
The position-coding pattern can be designed in a plurality of ways. It has been proposed that symbols in the form of dots of different sizes can be used, a large dot coding a “one” and a small dot coding a “zero”. Such a pattern is described in WO 00/73983, which is incorporated herewith by reference. Alternatively, dots with different displacements in relation to a raster can code different symbol values. Such patterns are described in WO 01/16691, PCT/SE00/01895 and PCT/SE00/01897, which are incorporated herewith by reference.
Such a position-coding pattern can code unique positions on a very large surface with good resolution. It is calculated that 4.6 million km2 can be coded with unique positions with good resolution.
In many cases, bases with a position-coding pattern can be generated on a large scale and with high precision in the graphics industry. This applies, for example, in the production of diaries or notepads. There are, however, also occasions when it is desirable to create bases with a position-coding pattern on a small scale. This can then be carried out using a personal computer, to which a printer of, for example, the ink-jet or laser type, has been connected.
This can be carried out in such a way that the required part of the position-coding pattern is created as a file in a graphical format, for example, in bmp format (bmp=bitmap). This graphical file is converted into a page-describing and normally printer-independent code, such as POSTSCRIPT (trademark), after which it is transmitted to the printer unit. On the basis of the page-describing code, the printer unit creates corresponding information which is used to control the printer's hardware. This hardware can, in the case of a laser printer, comprise a laser diode with associated optics. Where the printer is an ink-jet printer, the hardware comprises an ink ejector. The printout is then carried out on a base, for example a sheet of paper.
A problem associated with this method is that in the resultant printout the system introduces something that could be called “noise”. This noise arises as a result of, among other things, quantizing, and results in small displacements and/or deformations of the symbols in the position-coding pattern. The quantizing arises when the graphical file is adapted to the page-describing code and/or when the page-describing code is adapted to the printers hardware and software.
When printing out graphical files that describe other than position-coding patterns, for example images intended to be visible to the human eye, this does not normally cause problems. Small displacements or deformations of small elements in an image are not generally detected.
For a reading device of the above-mentioned type, which is intended to detect positions in a position-coding pattern, however, such displacements or deformations of the symbols in the pattern can cause problems, in that the reading device detects an incorrect or invalid position. This can occur if the resolution of the position-coding pattern is too high in relation to the performance of the printer and the quality (for example, the smoothness of the surface) of the base.